1. Field of the Invention
The present invention relates to a lithographic projection apparatus and device manufacturing method.
2. Description of the Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to device that cart be used endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny minors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U. S. Pat. No. 5,229,872. As above, the support in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (IC's). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the beam of radiation in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay (juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as “alignment system”), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel or preparatory steps may be carried our on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.
Optical elements for use in the extreme ultraviolet (EUV) spectral region, e.g. multi-layered thin film reflectors, are especially sensitive to physical and chemical damage which can significantly reduce their reflectivity and optical quality. Reflectivities at these wavelengths are already low compared to reflectors at longer wavelengths which is a particular problem because a typical EUV lithographic system may have 11 mirrors; four in the illumination optics, six in the imaging optics plus the reflecting reticle. There may also be a number of gracing incidence mirrors. It is therefore evident that even a “small” decrease of 1-2% in the peak reflectivity of a single mirror will cause a significant light through-put reduction in the optical system.
Conventional reflection optics mainly consist of Mo/Si multi-optical-layer optical elements for normal incidence reflection. Such multi-optical-layer optical elements have a theoretical reflectivity of about 73.7% (in practice values are about 69%) if they are Si terminated i.e. the outermost optical-layer is one of Si (not taking account of surface roughness or interfacial diffusion).
A problem exists with such optical elements due to high stresses in the multilayers as a result of large lattice mismatch between the Mo body centred cubic (BCC) crystal structure and that of the Si crystal structure (diamond structure). The stresses or stress relaxation can lead to early failure of the optical element or altering, respectively.
Also, from a theoretical standpoint, a Mo terminated multi-optical-layer optical element has a reflectivity approximately 1 to 1.5% higher than the conventional optical element, if the thickness of the outer Mo layer is 1.5 to 3 nm. However, after manufacturing optical elements, the outer surface is normally exposed to air during the assembly of the lithographic projection apparatus. On exposure to air, the outer Mo layer is completely oxidized and results in a dramatic loss of reflectivity of a few %, thus, to date, only multi-optical-layer optical elements which have an outer layer of Si are in use.
EUV radiation combined with molecular contamination due to water and oxygen (the later might be used for carbon mitigation or cleaning purposes) in the vacuum of an EUV scanner results in a dramatic reduction in the reflectivity of multi-optical-layer optical elements during use in a lithographic projection apparatus. Thus, optical elements have a short lifetime in EUV lithographic apparatus. This reduction in reflectivity, which is a result of oxidation of the outer surface, can occur at pressures as low as 10−7 mbar of water in the unbaked vacuum used in EUV scanners.
Recently it is has been shown experimentally (see European Patent Application No. EP 1,065,532) that a Ru cap layer as well as a dense packed C cap layer can reduce oxidation and hence also reflectivity loss. However, a thick Ru layer (of about 3 nm) causes an unacceptable reflection loss due to absorption whereas a thin layer (about 1.5 nm) results in intermixing with the layers on which the Ru is coated and therefore oxidation and also reflectivity loss. If a Ru protective outer layer of 1.5 m thickness is used with a 2.0 nm Mo layer, oxidation appears to be prevented for about 50 hours but the theoretical reflectivity of such an optical element is as low as 71.5% i.e. 2-3% lower than the standard Si terminated multi-optical-layer optical element.